Optical modulators with high modulation speed, small footprint, and large optical bandwidth are needed as the enabling device for on-chip optical interconnects.
Semiconductor optical modulators have witnessed rapidly-expanding research interests over the last few years. However, it has been found that the prior art semiconductor-based electro-optical modulators have disadvantages, including stringent fabrication tolerance, high cost, large device footprint, and high optical loss.
FIG. 1 depicts a prior art semiconductor optical modulator.
The device footprint of silicon-based modulators is on the order of millimeters, with these devices being limited by their weak electro-optical properties. Germanium and compound semiconductors, on the other hand, face the major challenge of integration with existing silicon electronics and photonics platforms. Integrating the silicon modulators with high quality-factor optical resonators efficiently increases the modulation strength. However, these devices suffer from intrinsic narrow-bandwidth, aside from their sophisticated optical design, stringent fabrication, and temperature tolerances. Notably, such semiconductor optical modulators are also polarization sensitive. Finding a complementary metal-oxide-semiconductor (CMOS) compatible material with adequate modulation speed and strength is becoming a task of not only scientific interest, but also industrial importance.
Graphene, a single layer of carbon atoms, has attracted growing attentions due to its outstanding and intriguing properties. Possessing the highest carrier mobility of more than 200,000 cm2/(V·s), graphene has stirred up particular interest for high-speed electronics, and is considered as a promising replacement for silicon for on-chip integration.
Graphene also shows attractive optical properties over a broad spectral range from the visible to mid-infrared (IR). A graphene-based plasmonic device at the mid-infrared (IR) regime has been recently explored, and innovative transformation optics have also been proposed on a graphene platform. Graphene can absorb 2.3% of the normal incident ultraviolet and visible light, despite the fact that it has only one atomic layer. This universal absorption coefficient is due to the unique linear and gapless band dispersion of Dirac fermions. Although this absorption is small, novel passive optoelectronics including mode-lock laser, polarizers, and photodetectors have already been demonstrated by utilizing the anisotropic absorption property of graphene and the generated hot electrons.
Graphene can also be actively tuned in a dramatic way. With the free electrons tightly confined within the single atomic layer, graphene has a very low density of states, especially when electron energy is close to the Dirac point. Slight variations of carrier density can therefore cause significant shifts in Fermi energy (EF) (the highest energy level of electrons), which changes the rate of interband transitions and subsequently the optical constant.
Therefore, an optical high performance, low insertion loss, graphene modulator is needed.